The ocean teems with diverse life forms capable of photosynthesis. While the term “plants” often suggests rooted, leafy structures, in the marine realm, it encompasses a wide array of organisms that convert sunlight into energy. These photosynthetic inhabitants form the base of marine food webs and play a substantial role in global oxygen production.
True Marine Plants
True marine plants are flowering plants, or angiosperms, adapted to saltwater environments. These species have evolved specialized features to survive and reproduce while fully submerged or in intertidal zones. They possess roots, stems, and leaves, absorbing nutrients through their root systems.
Seagrasses
Seagrasses form underwater meadows in shallow coastal waters and estuaries. Examples include eelgrass (Zostera marina) and turtle grass (Thalassia testudinum), which provide shelter and food for marine animals. Their extensive root systems stabilize sediments, preventing coastal erosion and maintaining water clarity. These plants also filter water and serve as nurseries for various fish and invertebrate species.
Mangroves
Mangroves are trees and shrubs flourishing in the intertidal zones of tropical and subtropical coastlines. They adapt to challenging habitats with specialized root systems, like prop roots or aerial pneumatophores, which absorb oxygen from anoxic soils. Mangroves manage high salt levels through ultra-filtration in roots or by excreting salt through leaf glands. These forests offer habitats, protect coastlines from erosion, and act as nurseries for marine and terrestrial species.
Seaweeds
Seaweeds are large, multicellular algae, not considered true plants because they lack vascular tissues, roots, stems, and leaves. They absorb nutrients directly from the surrounding seawater across their entire surface. Seaweeds anchor themselves to rocky surfaces using a holdfast, which serves solely for attachment.
The main body of a seaweed, called a thallus, consists of a stem-like stipe and leaf-like blades or fronds, where photosynthesis occurs. Many species, especially larger ones like kelp, feature gas-filled bladders that provide buoyancy, allowing blades to float closer to the sunlit surface. Seaweeds are categorized into three main groups based on their dominant photosynthetic pigments: red algae (Rhodophyta), brown algae (Phaeophyta), and green algae (Chlorophyta).
Red algae, found in deeper waters, possess pigments like phycoerythrin that capture blue-green light. Brown algae, including kelps, are characterized by the pigment fucoxanthin and form extensive underwater forests in cooler temperate regions. Green algae inhabit shallower waters and are the most diverse. These macroalgae provide food and shelter within marine ecosystems.
Microscopic Algae
Microscopic algae, predominantly phytoplankton, are photosynthetic organisms that drift within the water column. These single-celled or colonial organisms are abundant and diverse, forming the foundation of nearly all marine food webs. They produce a substantial portion of the Earth’s oxygen, estimated at around 50% globally.
Key groups of phytoplankton include diatoms, dinoflagellates, and coccolithophores. Diatoms are distinguished by their silica-based cell walls, called frustules, and convert sunlight into energy. Dinoflagellates possess two flagella that allow for limited movement; some species cause harmful algal blooms or exhibit bioluminescence. Coccolithophores are encased in plates made of calcium carbonate, known as coccoliths, which contribute to ocean sediments upon death.
These microscopic producers inhabit the sunlit surface layers of the ocean, known as the euphotic zone, where light allows for photosynthesis. They play a role in global biogeochemical cycles, including the carbon cycle, by absorbing carbon dioxide from the atmosphere. Phytoplankton are important in sustaining marine life and regulating Earth’s climate.